Nuclear magnetic resonance (NMR) imaging has been developed to a level where a number of machines are now being designed for clinical studies. Although it is thought that the energy delivered to the body with this modality is insufficient to cause any adverse biological effects, there are definite currents induced that could conceivably affect function of the nervous system or the heart. There have been attempts made to calculate the risk of adverse effect upon cardiac excitability, but the actual distribution or induction of currents in the heart due to static or dynamic magnetic fields, and the influence of cardiac and/or blood motion within these fields has not been directly measured. Preliminary experiments suggest that normal hearts in rather low strength magnetic fields do not change their heart rate nor are arrhythmias observed. Direct measures of fibrillatory propensity, or of cardiac excitability in relation to cardiac motion in the field have not been reported. Furthermore, physiologic stresses, pharmacologic agents, and pathologic states may significantly alter cardiac vulnerability. Our first aim is to define the changes in cardiac excitability that occur in very powerful static magnetic fields (1.9 T). We will directly measure fibrillation threshold and strength-interval curves to quantify cardiac excitability. We will also use canine models of enhanced cardiac vulnerability (digitalis toxicity, global cardiac ischemia) to detect adverse effects of static magnetic fields. Our second aim is to determine the electrical current and waveforms required to produce cardiac arrhythmias from mid-cavity stimulation sites in normal and ischemic dog hearts. From this data, we will calculate the magnetic field fluctuations that would be required to produce such adverse currents in the myocardium. Finally, where potential adverse effects have been suggested by the foregoing, we will verify such events with magnetic fields of appropriate intensity and waveform in suitably sensitive animal models. Based upon these studies, it should be possible to estimate the risks of inducing arrhythmias or fibrillation in patients who are placed in resistive or supercooled magnets for NMR images.